A Cassyrane derivative with a puckered ring (top) has a stronger odor than a compound with a flat ring (bottom).

Credit: Angew. Chem. Int. Ed.

A Cassyrane derivative with a puckered ring (top) has a stronger odor than a compound with a flat ring (bottom).

Credit: Angew. Chem. Int. Ed.

The secret behind the trendy aroma of black currant may be all about puckering up—five-membered ring puckering, that is. A team at Swiss fragrance firm Givaudan and RWTH Aachen University in Germany has shown that the structure and shape of an ingredient in black-currant scents influence its olfactory properties (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201100937). Black currant, or cassis in French, is a berry from a plant native to Europe and Asia, and its scent has enjoyed a new surge of popularity with the success of products such as DKNY Be Delicious perfume. The cassis aroma is highly dependent on the stereochemistry of its substituent compounds, and Givaudan chemist Philip Kraft wanted to see whether that held true for Cassyrane, a proprietary black-currant odorant introduced by Givaudan in 2010. With Aachen’s Wolfgang Stahl and colleagues, he combined quantum chemistry calculations with microwave spectroscopy to determine gas-phase structures of Cassyrane stereoisomers and derivatives. They found that stereochemistry at Cassyrane’s number five carbon determines whether the molecule smells like cassis, and that puckering of the odorant’s furan ring enhances its fruity cassis character.